This invention relates generally to optical scanning systems, and, more particularly, to optical scanning systems that provide a raster scan of a collimated beam of light across a surface, as part of a system for reading and/or recording images.
Optical scanning systems of this particular kind are now in common use in numerous applications, including laser printing and xerography. In the case of laser printing, an intensity-modulated laser beam typically is made to scan repeatedly along the longitudinal axis of a cylindrical photoreceptive drum, in synchronism with a controlled rotation of the drum. This forms a raster-scan image on the drum, which can thereafter be transferred to paper.
The repeated longitudinal scanning of the laser beam on the photoreceptive drum is typically accomplished using a movable mirror such as a polygonal mirror assembly or a resonating galvanometric mirror. As soon as each scan has been completed, the next scan begins, by which time the drum will have been rotated a prescribed amount.
Imperfections in the movable mirror, such as a pyramidal misalignment of one or more mirrors of the polygonal mirror assembly, can cause the scanning beam of light to deviate transversely from its intended direction, in a cross-scan direction. A distorted, non-uniform raster scan image is thereby produced.
Efforts in the past to correct for cross-scan deviation errors of this kind have centered largely on improving the construction of the polygonal mirror assembly, itself, and on providing an additional lens system for redirecting the deviated beam back toward its desired point of impingement on the drum. Although these efforts have generally proven effective in providing a raster scan image of a desired precision and accuracy, the solutions are believed to have required undue expense and complexity.
It should, therefore, be appreciated that there still is a need for an improved optical scanning system that precisely aligns a scanning beam with a photosensitive drum, without requiring the use of a relatively expensive and complex lens structure. The present invention fulfills this need.